The present invention relates to an ultrasonic transducer and, more particularly, to an ultrasonic transducer suitable for use in an obstacle detection device for automobiles capable of detecting any obstacle which may lie ahead or behind the running automobile.
Aerial ultrasonic transducer incorporating a bimorph cell have been put into practical use. One of these known transducer has two keep plates between which the bimorph cell is clamped. In this type of transducers, there is a fear that the bimorph cell may come off from the keep plates due to an externally applied impact. In addition, audible noises are generated from the points on the keep plates in a position where the bimorph cell is supported, when the transducer is used for the transmission of wave. It is also to be pointed out that this type of transducer has a rather inferior sensitivity.
In order to obviate these drawbacks, there has been proposed a horn type ultrasonic transducer in which a disc-shaped bimorph cell is supported along the nodal circle thereof and a coupling cone is inserted into a central hole of the disc-shaped bimorph cell. Also, a metallic horn type resonator is bonded to the bimorph cell. Due to the attaching of the metallic resonator, the bimorph cell makes a vibration at its central portion at a large amplitude when an ultrasonic wave is applied at the neutral frequency thereof, so that a comparatively high output voltage is derived from the peripheral part of the bimorph cell.
This horn type ultrasonic transducer, however, has the following problem when it is mounted on an automobile as an obstacle detecting measure. Namely, rainwater, as well as water for washing the automobile, tends to come into the horn type resonator so that many water droplets attach to the bimorph cell and the horn type resonator due to the surface tension to seriously deteriorate the wave transmitting and receiving sensitivities. The attaching of the water droplets cannot be avoided even when the transducer is directed horizontally or downwardly.